Combustion of hydrocarbon fuel, such as in an internal combustion engine, produces exhaust gas composed of a mixture of constituent gases, including oxide gases (e.g., NOX, SOX, CO2 and CO), unburned hydrocarbons and O2. It is advantageous to accurately measure the concentration of one or more of these constituent gases in the exhaust gas in real time. For example, accurate, real time concentration measurements of NOX gas can be used as part of a feedback loop for ensuring that the emissions of NOX gas from an internal combustion engine are maintained at an acceptable amount.
U.S. Pat. No. 7,217,355 issued to Nair et al. on May 15, 2007, discloses a gas sensor for detecting concentration of NOX gas in a stream of gas. The sensor includes a cylindrical housing with an open end constituting an inlet through which a stream of gas enters the sensor. An input assembly including a catalyst assembly is secured in the housing adjacent to the inlet. A heating rod is used to heat the sensor to an optimal operating temperature above the temperature of the gas stream. A NOX sensor electrode is received in the housing downstream from the input assembly. A gas exit port projects laterally from the housing near the NOX sensor electrode. The gas exit port comprises a small diameter stainless steel tube that, when connected to some type of suction device, will draw the exhaust gas stream through the input assembly and out the housing through the exit port. The suction device can be a small air pump, or the gas suction can be accomplished using the vacuum lines commonly implemented in internal combustion engines.
Requiring a pump or prime mover to pump gas through the sensor makes the sensor more difficult to install in a machine, particularly an exhaust system of a machine, and makes operation of the sensor dependent on other components of the machine, which can make troubleshooting more difficult. Moreover, where the housing is inserted in an exhaust tube and designed to sample the exhaust stream passing over the housing, the relatively cool exhaust gas will cool the components in the housing, requiring more power to drive the healing rod to maintain the sensors at the optimal operating temperature.